1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device which has a planarly dispersed charge storing means (for example, in a MONOS type or a MNOS type, charge traps in a nitride film, charge traps near the interface between a top insulating film and the nitride film, small particle conductors, etc.) in a gate insulating film between a channel forming region and a gate electrode in a memory transistor and is operated to electrically inject primarily channel hot electrons, ballistic hot electrons, secondarily generated hot electrons, substrate hot electrons, and hot electrons caused by band-to-band tunneling current into the charge storing means to store the same therein and to extract the same therefrom and a method for operating the device.
2. Description of the Related Art
Nonvolatile semiconductor memories offer promise as large capacity, small size data-storage media. Along with the recent spread of broadband information networks, however, write speeds equivalent to the transmission rates of the networks (for example, a carrier frequency of 100 MHZ) are being demanded. Therefore, nonvolatile memories are being required to have good scaling and be improved in write speed to one or more order of magnitude higher than the conventional write speed of 100 μs/cell.
As nonvolatile semiconductor memories, in addition to the floating gate (FG) types wherein the charge storing means (floating gate) that holds the charge is planarly continuously spread in a plane, there are known MONOS (metal-oxide-nitride-oxide-semiconductor) types wherein the charge storing means are planarly dispersed.
In a MONOS type nonvolatile semiconductor memory, since the carrier traps in the nitride film [SixNy (0<x<1, 0<y<1)] or on the interface between the top oxide film and the nitride film, which are the main charge-retaining bodies, are spatially (that is, in the planar direction and thickness direction) dispersed, the charge retention characteristic depends on not only the thickness of a tunneling insulating film, but also on the energy and spatial distribution of the charge captured by the carrier traps in the SixNy film.
When a leakage current path in locally generated in the tunneling insulating film, in an FG type, a large amount of charge easily leaks out through the leakage path and the charge retention characteristic declines. On the other hand, in an MONOS type, since the charge storing means is spatially dispersed, only the charges near the leakage path will locally leak from it, therefore the charge retention characteristic of the entire memory device will not decline much.
As a result, in a MONOS type, the disadvantage of the degradation of the charge retention characteristic due to the reduction in thickness of the tunnel insulating film is not so serious as in an FG type. Accordingly, a MONOS type is superior to an FG type in scaling of a tunneling insulating film in a miniaturized memory transistor with an extremely small gate length.
Moreover, when a charge is locally injected into the plane of distribution of the planarly dispersed charge traps, the charge is held without diffusing in the plane and in the thickness direction, the contrary to an FG type.
To realize a miniaturized memory cell in a MONOS type nonvolatile semiconductor memory, it is important to improve the disturbance characteristic. Therefore, it is necessary to set the tunneling insulating film thicker than the normal thickness of 1.6 nm to 2.0 nm. When the tunneling insulating film is formed relatively thick, the write speed is in the range of 0.1 to 10 ms, which is still not sufficient.
In other words, in a conventional MONOS type nonvolatile semiconductor memory etc., to fully satisfy the requirements of reliability (for example, data retention, read disturbance, data rewrite, etc.), the write speed is limited to 100 μs.
A high speed is possible if the write speed alone is considered, but sufficiently high reliability and low voltages cannot be achieved. For example, a source-side injection type MONOS transistor has been reported wherein the channel hot electrons (CHE) are injected from the source side (IEEE Electron Device Letter, 19, 1998, p. 153). In this source-side injection type MONOS transistor, in addition to the high operation voltages of 12V for write operations and 14V for erasure operations, the read disturbance, data rewrite, and other facets of reliability are not sufficient.
On the other hand, taking note of the fact that it is possible to inject a charge into part of dispersed charge traps area by the conventional CHE injection method, it has been reported that by independently writing binary data into the source and drain side of a charge storing means, it is possible to record 2 bits of data in one memory cell. For example, Extended Abstract of the 1999 International Conference on Solid State Devices and Materials, Tokyo, 1999, pp 522-523, considers that by changing the direction of the voltage applied between the source and drain to write 2 bits of data by injecting CHE and, when reading data, applying a specified voltage with a direction reversed to that for writing. By the so-called “reverse read” method, correct reading of the 2 bits of data is possible even if the write time is short and the amount of the stored charge in small. Erasure is achieved by injecting hot holes.
By using this technique, it becomes possible to increase the write speed and largely reduce the cost per bit.
However, in a conventional CHE injection type MONOS type nonvolatile semiconductor memory, since Electrons are accelerated in the channel to produce high energy electrons (hot electrons), it is necessary to apply a voltage of about 4.5V between the source and drain, and it in difficult to decrease this source-drain voltage. As a result, in a write operation, the punch-through effect becomes a restriction and good scaling of the gate length is difficult.